remove SkFixedMul_arm()
remove SkFixedMul_arm()

The portable SkFixedMul_longlong (here now renamed SkFixedMul)
generates shorter, equivalent code,

from
   0:   fb81 0200       smull   r0, r2, r1, r0
   4:   ea4f 4010       mov.w   r0, r0, lsr #16
   8:   ea40 4002       orr.w   r0, r0, r2, lsl #16

to
   0:   fb81 0100       smull   r0, r1, r1, r0
   4:   0c00            lsrs    r0, r0, #16
   6:   ea40 4001       orr.w   r0, r0, r1, lsl #16

(Notice, 2 bytes saved.)

Change-Id: Icb0f7e6d4379086fc602f956a4beb1265a9759bc
Reviewed-on: https://skia-review.googlesource.com/69440
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>

Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,HalCanary/skia-hc,google/skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,rubenvb/skia,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,rubenvb/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc